Cutin, the structural component of the plant cuticle, is a polyester. Natural cutin polymeric arrays are found in the outer surfaces of leaves and shoots of plants. In natural processes, cutin, a non-living substance, results from the polymerization of fatty acids in the presence of oxygen.
Cutin is the major constituent (between 40% and 80% of weight) of the cuticle and is defined as a polymeric network of oxygenated C16 and C18 fatty acids cross-linked by ester bonds. Specifically, cutin is a polyester composed of ω-hydroxy-C16 and C18 fatty acids, dihydroxy-C16 acid, 18-hydroxy-9,10-epoxy-C18 acid and 9,10,18-trihydroxy-C18 acid. Cutin can be depolymerized by cleavage of the ester bonds by alkaline hydrolysis, transesterification and other methods. These chemical methods yield monomers and/or their derivatives depending on the reagent used. The 9- or 10,16-dihydroxyhexadecanoic acid and 16-hydroxyhexadecanoic acid are the major components of the C16 cutins. Only in some cases, 16-hydroxy-10-oxo-C16 acid and 16-oxo-9 or 10-hydroxy C16 acid are monomers. Major components of the C18 family of monomers are 18-hydroxy-9,10-epoxyoctadecaonoic acid and 9,10,18-trihydroxyoctadecanoic acid together with their monounsaturated homologues. Generally, cutin polyester is held together by primary alcohol ester linkages with about half of the secondary hydroxyl groups involved in ester cross-links (Heredia, A. 2003)
In the cuticle of some plant species, once all the wax and cutin components have been removed, there is some remaining residual material. This depolymerization-resistant residue is believed to represent cutin monomers held together by nonester bonds and is termed cutan. While the cuticles of some plant species appear to completely lack cutans, in a number of species, the two biopolymers, cutin and cutan, may occur in varying ratios according to their relative abundance at different stages of cuticle development. (Heredia, A. 2003)
Heredia-Guerrero et al. have synthesized a memetic polymer of plant cutin from 9,10,16-trihydroxyhexadecanoic (aleuritic) acid through a low temperature polycondensation reaction. In this synthetic polymer, the polyaleurate framework was found to be more rigid than natural cutin having additional larger short-range ordered domains and displaying slightly different mechanical properties with respect to natural cutin due to additional hydrogen bonding within the framework of polyaleurate. (Heredia-Guerrero, J. A. et al., 2009)
Essential synthetic issues associated with the production of cutin in plants have been studied in botany and plant lipid biochemistry. (Franke, R. et al., 2005; Li, Y. et al., 2007; Li-Beisson, Y. et al., 2009; Panikashvili, D. et al., 2010; Pollard, M. et al., 2008; Riederer, M. et al., 1993) In some instances, these fields recognized different derivatives of cutin monomers with individual names. For instance, cutan is a non-hydrolysable aliphatic biopolyester found in the Agave americana. (Gupta, N. et al., 2006) Suberin, which is also derived from cutin monomers found in Cereus peruvianus cacti, is synthesized from aliphatic and phenolic polyesters. (Franke, R. et al., 2007; Kim, Y. H. et al., 2002) Cacti also have surface waxes formed by epicuticular lipids which are randomly structured. (Maiti, R. K. et al., 2003; Rezanka, T. et al., 1998)
In particular, the composition of cutin found in the outer layer of cactus pads and shoots is of interest in 2-D layered technology development. Cactus epidermis and cuticles are capable of withstanding UV radiation damage over long periods of time (>100 years), and they tolerate relatively high and low temperatures, endure abrupt changes of temperature, withstand water damage, and reflect light. (Drezner, T. D., 2008) Natural cutin surfaces are extremely stable, as they do not crack nor corrode like other synthetic materials under the same extreme weather conditions. Moreover, cutin-like materials can be engineered to be amorphous or crystalline and readily impregnated with metals and metal oxides materials. If crystalline surfaces are wanted, the polyesters are aligned before curing in crystallographic configurations by using shearing forces or conditions for self-assembly. This leads to regions where the polymers are ordered in a specific direction. If amorphous materials are wanted, the polyesters are cured after being mixed. This leads to regions with random configurations.